This invention relates generally to a liquid dispensing apparatus which is particularly, although not exclusively, suitable for use in the fuser of an electrostatographic recording machine such as, for example, a xerographic copier.
In a xerographic copier a light image of an original document to be reproduced is recorded in the form of a latent electrostatic image on a photosensitive member. The latent image is rendered visible by the application of a resin-based powder known as toner. The visual toner image is transferred electrostatically from the photosensitive member on to sheets of paper or other substrates. The toner image is then fixed or "fused", for example by applying heat and pressure, which causes the toner material to become soft and tacky whereby it is able to flow into the fibers or pores of the substrate or otherwise upon the surface thereof. Thereafter, as the toner material cools, it solidifies and is bonded firmly to the substrate. In the electrostatographic art generally the use of thermal energy and pressure for fixing toner images on to a substrate is well known.
It has long been recognized that one of the fastest and most positive methods of applying both heat and pressure for fusing the toner image to the substrate is by direct contact of the resin-based toner image with a hot surface such as a heat roller which also applies pressure to the substrate. One approach is to pass the substrate with the toner image thereon between a pair of opposed rollers forming a nip, at least one of the rollers being internally heated. The actual temperature and pressure ranges will of course vary depending upon the softening range of the particular resin used in the toner. Typically, however, it will be necessary to heat the toner powder above 180.degree. C. Temperatures of 200.degree. C. or even higher are not uncommon in commercial fusers.
A problem with this kind of fuser is that, as the toner becomes tacky, it can stick to the surface of the fuser roller which is undesirable because some of the toner on the fuser roller can then be transferred to subsequent substrates being fused. This effect, known as "offset", clearly impairs copy quality. Furthermore, if the rollers are rotated when there is no substrate present in the nip therebetween, toner may also be transferred from the fuser roller to the backup roller so that when a substrate subsequently passes through the nip some of the toner may be transferred to the reverse side thereof. Also, stripping failures may occur because substrates may stick to the rollers instead of simply passing between them.
An arrangement for minimizing the problem of offset has been to provide a fuser roller with an outer surface or covering of, for example, polytetrafluoroethylene known by the trade name Teflon, to which a liquid release agent such as silicone oil is applied. The thickness of the Teflon is typically of the order of tens of microns and the thickness of the oil is less than 1 micron. Silicone based oils, for example polydimethylsiloxane, which possess a relatively low surface energy, have been found to be suitable for use in the heated fuser roller environment where Teflon constitutes the outer surface of the fuser roller. In practice, a thin layer of silicone oil is applied to the surface of the heated roller to form an interface between the roller surface and the toner images carried on the substrate. Thus, a low surface energy layer is presented to the toner as it passes through the fuser nip thereby preventing toner from offsetting to the fuser roller surface.
In attempts to improve the quality of the image fused by a heat roller fuser, such rollers have been provided with conformable surfaces comprising silicone rubber or Viton (Trademark of E I Du Pont for a series of fluoroelastomers based on the copolymer of vinyladinefluoride and hexafluoropropylene). As in the case of the Teflon coated fuser roller, release fluids such as silicone based oils are applied to the surface of the silicone rubber or Viton to both minimize offsetting and to facilitate stripping. When the fuser system is one which provides for applying silicone oil to silicone rubber or Viton, a low viscosity silicone oil (i.e. in the order of 100 to 1000 centistokes) has most commonly been employed, although liquids of relatively high viscosity, for example 12,000 to 60,000 centistokes and higher, have also been used.
Various forms of applicator have been employed to supply the liquid release agent to the surface of the fuser roller. Thus, for example, U.S. Pat. No. 4,231,653 discloses an applicator comprising an elongate trough for containing a supply of release oil. A wick which is partially immersed in the release oil supply draws the oil up from the trough for application to the fuser via a pair of cooperating rollers in pressure contact, namely a driven oil application roller and a freely rotatable oil supply roller. The wick is in engagement with the oil supply roller and thus applies the release oil directly to the surface thereof. The oil supply roller slips on the application roller and is not rotated when there is some oil present between the two rollers, but as the oil runs out the oil supply roller is driven by the oil application roller since the coefficient of friction therebetween is increased. In other words, the oil supply roller is rotated only when there is little or no oil on the surface of the oil application roller due to the application of oil to the fuser and thus the cooperating roller pair acts as a metering device for checking the amount of release oil conveyed to the fuser.
In order to supply the release oil to the trough in an applicator of the kind just described, various devices have been used or proposed. In one arrangement, a peristaltic pump was used, requiring a drive motor, or a separate drive derived from one of the motors already in the copying machine. Because of the tiny amounts of release oil that have to be supplied, of the order of a few milliliters per day, a reduction gearing arrangement was also needed. This added to the complexity and cost of the machine. A simple alternative arrangement for supplying fluid to a controlled level is the inverted bottle or "chicken feeder" device. However, due to space constraints, when such a device has to be mounted near the fuser, problems can arise. Heat from the fuser causes air within the bottle to expand and drive excessive amounts of oil into the trough resulting in spillage.
U.S. patent application Ser. No. 140,179 filed December 31, 1987 discloses a release agent management (RAM) system for applying silicone oil to a heated fuser roll. The RAM comprises an airtight container which uses heated air to pump silicone oil from the container into a trough. The energy for heating the air is either ambient heat or a heating element disposed in the container.
Thus neither of these systems is entirely satisfactory for a compact and low cost copier, and it is an object of the present invention to meet the need for a low cost but reliable system for delivering release oil to a fuser.
Japanese publication No. 58-200264 published on November 21, 1983 discloses an oil applying device which utilizes a heater to effect a pressure rise in a container for causing a valve to open and close for allowing oil to flow from the container.
U.S. Pat. No. 3,180,278 issued on Apr. 27, 1965 discloses a fluid pump which uses a heater for transferring fluids from one vessel to another.
U.S. Pat. No. 4,512,650 discloses a heat and pressure fuser utilizing a release agent management system including a fuser oil supply bottle.
U.S. Pat. No. 4,087,676 issued on May 2, 1978 discloses a heat and pressure fuser apparatus including a wick for applying release agent material to a fuser roll.